The invention relates to the field of mobile phone systems. A subscriber in a mobile phone system, i.e. a subscriber station, for instance a mobile phone or a mobile station, may be registered in a radio network or system, whereby it communicates with the radio network via system channels comprised of control and traffic channels maintained by the base stations of the radio network.
In addition to the system channels in mobile phone systems, so-called direct mode channels can also be used in connection with a radio system, i.e. direct mode operation is applied. Subscriber stations using direct mode operation do not communicate directly with the radio network or its base stations. Direct mode channels are frequencies on which mobile phones or other means of communication are able to communicate directly with each other without the system, or via repeater stations either with the base stations of the system or with other mobile stations.
Direct mode channels are typically used in situations where, for instance, a group of portable mobile phones are communicating with each other at such a long distance from the base station that system channels cannot be used.
Another important way of using direct mode channels is to increase the capacity when the traffic in the system increases rapidly in some part of the service area of the system, for instance in some point-like part of the radio network.
A direct mode channel is referred to with the terms direct or simplex channel, or a simplex connection. A direct mode channel is a channel which is typically not at all used by the system. For example, it may be a channel with the same channel spacing as the channels of the system, for instance 12.5 kHz or 25 kHz. Among the mobile phones operating on a direct mode channel, the transmitting station has tuned its transmitter to the channel and transmits speech or data information. The other mobile phones set to direct mode operation have tuned their receivers to the same channel, whereby they are able to directly hear the transmission.
Operation on a direct mode channel may take place on the analog or digital modulation principle. A mobile phone transmitting on the channel may also transmit signalling information, such as information on rights of use and priorities or on the group operating on the channel. On the direct mode channel, an encryption may be carried out or plain speech can be transmitted.
Subscriber stations using direct mode operation communicate with other subscriber stations on a direct mode channel without necessarily being in direct contact with the base stations of the radio network. However, the subscriber stations using direct mode operation can also communicate with the radio network via repeater stations. A repeater station is a piece of equipment comprising two radio apparatuses connected to each other. The repeater stations typically comprise two transceivers. The repeater station transmits the information messages transmitted on the direct mode channel to desired network elements, for instance to the base stations of the radio network, these base stations transmitting the information messages further to the switching centres of the radio network. The information messages transmitted by the repeater station may, for example, be speech or consist of data or signalling messages. When a repeater is used, the direct mode channel employs, for example, a semiduplex mode of operation. It should be noted that the repeater station may also be a repeater repeating traffic between mobile stations communicating on the direct mode channel.
It should be noted that, in direct mode operation, the repeaters/repeater stations may also repeat traffic between two mobile stations located within the coverage area of each respective repeater, but not necessarily within the coverage areas of each other.
The U.S. Pat. No. 5,056,152 discloses a vehicular repeater operating as a repeater between a base station and a mobile station of the system. However, the solution described in that U.S. Patent is designed to transmit (i.e. to repeat) on a different frequency than on which the mobile station whose messages are to be repeated is transmitting. Therefore, the solution disclosed in that U.S. Patent is not suitable for repeating/transmitting traffic between mobile stations communicating with each other on the same direct mode channel. According to the solution in accordance with that U.S. Patent, a check is carried out as to whether the repeater frequency is free, by transmitting a signal (tone) on the transmitting frequency of the mobile stations and then monitoring on the repeater frequency whether other repeaters repeat the signal on that frequency or not. In that U.S. Patent, a repeater 2 transmits a signal (tone) on a frequency A, and monitors on a frequency B how many other repeaters repeated that signal. On the basis of this, the repeater 2 calculates a delay V to employ. When a mobile station begins a speech transmission on the frequency A, the repeater 2 is ready to begin repeating the speech transmission on the frequency B after the delay V from the first possible moment of repeating. If another repeater begins repeating before the start of transmission of the repeater 2, the repeater 2 does not repeat the speech but stays in reserve.
The solution according to the above-identified U.S. Patent even forces a repeater reserved for another group to indicate its presence to an active repeater. The same holds true for a repeater belonging to the same group, intending to switch onto a second direct mode channel or to operate as an ordinary mobile station.
A problem in the prior art solutions is that the test transmission (tone) transmitted by the repeater, i.e. the signal for examining whether there are other repeaters on the channel besides the examining one, makes it imperative that the mobile stations be able to interpret the signal in a correct way, that is, to do nothing, because the signal is only a test transmission from a repeater seeking to switch onto the direct mode channel. Furthermore, such a test transmission permits collision of the signal from the repeater and the call set-up of the mobile station. Yet another problem emerges if the repeater seeking to switch onto the channel is transmitting its test transmission, and there are several other repeaters on the channel, all of which repeat the test transmission. This may easily lead to the entire capacity of said channel being reserved for the transmission of test transmissions and the repeated test transmissions. Such a situation is even worse on a direct mode channel having one frequency only.
While operating on a direct mode channel, the repeater station repeats and the mobile stations transmit on the same frequency. Consequently, to employ this frequency for selecting an active repeater station according to the method of U.S. Pat. No. 5,056,152 is not advisable, because that would interfere with the signalling of mobile stations on the direct mode channel.
In case a repeater seeking to switch onto the channel transmits, as in that U.S. Patent, several test messages that the other repeaters repeat, the resulting overlapping operation on the direct mode channel is problematic, particularly in a time division digital system in which the detection of overlapping transmissions is more difficult than in an analog system. The fact is that if two active repeaters accidentally transmit or repeat the test message in the same time slot, the transmission from both the repeaters may be destroyed, and the repeater seeking to switch onto the direct mode channel erroneously infers that there is no repeater on the direct mode channel. In practice, simultaneous operation as repeaters would result in the mobile stations being unable to decode transmission from either of the repeater stations.
The solution to the problems discussed above is to prevent the developing of such problematic situations by the repeater station seeking to switch onto the direct mode channel without checking that the direct mode channel is free for the repeater to operate.